This invention relates to a free piston heat engine driven alternator system, and more particularly to a stabilizing control for a power system including a free piston Stirling engine directly driving an oscillating alternator.
The free piston Stirling engine is a technology which has attracted considerable attention recently because of its high potential efficiency and multi-fuel capability. The engine can be hermetically sealed and permanently lubricated by gas bearings so that it may be designed for extremely long life and minimal periodic maintenance requirements.
Significant improvements have been made in this technology in the recent years, as evidenced by the power systems disclosed in U.S. Pat. application Ser. No. 168,714 for "Heat Engine Device" by Harlan V. White, and U.S. Pat. application Ser. No. 168,075, for "Stirling Engine with Diaphragm Coupling between Displacer and Power Piston," of Folsom and Dineen, both filed on July 14, 1980, and U.S. Pat. application Ser. No. 172,373 for "Diaphragm Displacer Stirling Engine Powered Alternator-Compressor," filed on July 25, 1980, by Lawrence R. Folsom, et. al., the disclosures of which are incorporated herein by reference.
Ironically, the great improvements introduced in this technology, as exemplified in the aforementioned applications, have given rise to an unexpected problem. It was found that under some conditions of heater head temperature, engine pressure, cooler temperature, and displacer spring stiffness, the system would become unstable. This was manifested by a tendency for the system to stall down to a very short stroke and very low alternator output, or to run to the opposite extreme with the piston violently striking the stops at both ends of its travel.
The great improvements made to the free piston Stirling engine in efficiency, durability, reliability, and many other aspects of its operation make this engine an ideal candidate for a host of applications including such diverse uses as residential heat pumps, power generators for recreational vehicles, irrigation pumps for remote areas, and power generation for electric vehicles, farm equipment, and space probes. However, in order for these potential applications to be realized it is necessary that the advanced, more sophisticated generations of this technology be stably operable under all conditions of load and load fluctuation.